1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit for optimizing performance of optical amplifiers.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Optical amplifiers are used to extend the distance of transmitted signals in fiber optic networks. This enables optical signals to be amplified without incurring the additional latency (and computer performance impact) of an optical-to-optical conversion. Optical amplifiers additionally offer lower timing jitter in some cases, and improved performance on long links.
In Wavelength Division Multiplexing (WDM) applications, optical amplifiers must be implemented to amplify many wavelengths spaced closely together at the same time. The basic function of the optical amplifier is to accept an input optical signal and amplify it without converting the signal to electrical form. An Erbium Doped Fiber Amplifier (hereinafter xe2x80x9cEDFAxe2x80x9d) is one type of optical amplifier that relies on stimulated emission of light at the proper signal wavelength. For example, as shown in FIG. 1, the basic EDFA 99 receives an input optical signal 120, which may consist of multiple wavelengths near the 1550 nm passband. This input signal 120 is passed through a circulator or optical isolator 130 to remove unwanted noise, and then enters via coupler 150, a section of optical fiber 170 several meters in length that is doped with Erbium ions. The principles of operation for the EDFA itself are well known: the Erbium is excited to an elevated energy state by a laser diode pump (LD pump) 110, similar to an optically pumped laser system. The LD pump 110 particularly generates a pump laser signal that is coupled to the erbium doped optical fiber 170 through coupler 150. The 1550 nm signal passing through this fiber produces stimulated emission of light at the same wavelengths as the 1550 nm signals, increasing their amplitude by up to 20 dB or more before the signals 190 exit the optical amplifier. By altering the doping of the erbium fiber 170, amplification can be obtained for industry standard C-band and L-band wavelengths. The amplifier gain is proportional to the intensity of the pump laser diode.
It is the case however, that EDFAs also produce background noise in the form of light which is not at the desired wavelength; this results from amplification of random photons outside the signal bandwidth, or spontaneous emission of photons within the EDFA which are subsequently amplified. This phenomena is known as Amplified Spontaneous Emission (ASE) noise. Most optical amplifiers have a strong ASE peak around a wavelength of 1533 nm, with weaker effects at other wavelengths. For this reason, commercial EDFAs are designed with a filter to remove ASE at this wavelength. However, as it is not possible to make an ideal bandpass filter at a specific wavelength, this approach does not remove all the ASE from an amplifier. Consequently, ASE builds up in a network with many stages of amplification, and is a limiting factor in the design of long links. As ASE is proportional to the amplifier gain, it would be highly desirable to provide a system and method for controlling and limiting ASE so that the gain of optical amplifiers may be increased resulting in increased link distances in optical networks.
As noted above, an EDFA operates on the same principle as an optically pumped laser; it consists of a relatively short (about 10 meters) section of fiber doped with a controlled amount of erbium ions. When this fiber is pumped at high power (10 to 300 mW) with light at the proper wavelength (e.g., 980 nm or 1480 nm wavelengths) the erbium ions absorb the light and are excited to a higher energy state. Another incident photon around 1550 nm wavelength will cause stimulated emission of light at the same wavelength, phase, and direction of travel as the incident signal. EDFAs are often characterized by their gain coefficient, defined as the small signal gain divided by the pump power. As the input power is increased, the total gain of the EDFA will slowly decrease; at some point, the EDFA enters gain saturation, and further increases to the input power cease to result in any increase in output power. Since the EDFA does not distort the signal, unlike electronic amplifiers, they are often used in gain saturation. The gain curve of a typical EDFA is not uniform over different wavelengths; for example, the gain at 1560 nm is about twice as large as the gain at 1540 nm. This can be a problem when operating wavelength division multiplexing (WDM) systems; some channels will be strongly amplified and dominate over other channels that are lost in the noise. Furthermore, a significant complication with EDFAs is that their gain profile changes with input signal power levels. Thus, for example, in a WDM system the amplifier response may become nonuniform (different channels have different effective gain) when channels are added or dropped from the fiber. As optical amplifiers do not amplify all wavelengths equally, some form of equalization is required in order to achieve a flat gain across all channels.
Current methods for providing gain equalization include adding an extra WDM channel locally at the EDFA to absorb excess power (gain clamping), and manipulating either the fiber doping or core structure. The most commonly used method today in commercial EDFAs is to manually adjust the gain whenever channels are added or dropped from the network, using variable optical attenuators. However, this type of manual intervention is not desirable in large, complicated networks.
It would thus be highly desirable to provide a system and method of automatic gain control (AGC) for optical amplifiers which automatically provides for the adjustment of the optical gain when there is a change in the power of the input signal.
Furthermore, it would be highly desirable to provide a system and method of automatic gain control (AGC) for optical amplifiers that enables dynamic adjustment of the optical output power of the pump laser diode in response to changing conditions on the input fiber link.
Moreover, it would be desirable to provide the capability of manually adjusting EDFA optical amplifier gain from a remote location.
It is therefore an object of the present invention to provide a system and method for automatically varying the pump laser power to the optical amplifier, and perform gain equalization for an optical amplifier using a feedback control loop.
It is another object of the present invention to provide a servo/feedback loop for an EDFA optical amplifier implemented in a WDM system, that enables dynamic adjustment of the pump laser power for performing gain equalization across one or more wavelengths in response to changing conditions on the input fiber link.
It is a further object of the present invention to provide a servo/feedback loop for an optical amplifier that enables locking of the pump laser center wavelength to the peak of the passband of an optical filter to reduce generation of unwanted ASE noise in optical amplifier systems.
It is yet another object of the present invention to provide a control mechanism for controlling gain of an EFDA optical amplifier from a remote location without having to make adjustments on the front panel of the optical amplifier product.
It is still another object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables dynamic gain control adjustment of an optical amplifier implemented in multi-gigabit laser/optical systems, thereby enabling significantly larger link power budgets and longer supported distances.
It is yet still another object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables dynamic gain control adjustment of an optical amplifier implemented in multi-gigabit laser/optical systems such that lower cost lasers and filters may be used providing a significant cost reduction in the equipment utilized.
Thus, according to one aspect of the invention, there is provided an optically pumped amplifier for amplifying optical signals comprising: an optical isolator device for receiving an input optical signal; a laser pump device for generating a laser pump signal having a peaked spectrum function including a center wavelength; an optical filter having a peaked passband function including a center wavelength implemented for receiving the laser pump signal and passing the laser pump signal at a pre-determined wavelength advantageous for amplifying the input signal; a doped fiber element for receiving the input signal and the laser pump signal and amplifying the input light signal in response to receipt of a laser pump signal at the pre-determined wavelength; and, a wavelength-locked loop servo-control circuit that enables real time mutual alignment of the laser pump signal center wavelength with the optical filter having the peaked passband function at the pre-determined wavelength, wherein the laser pump signal is maximally transferred to the erbium doped fiber thereby resulting in reduced noise characteristics of said amplifier.
According to another aspect of the invention, in an optically pumped amplifier including: an optical isolator device for receiving an input optical signal, a laser pump device for generating a laser pump signal having a peaked spectrum function including a center wavelength, an optical filter having a peaked passband function including a center wavelength implemented for receiving the laser pump signal and passing the laser pump signal at a pre-determined wavelength for amplifying the input optical signal; and, a doped fiber element for receiving the input signal and the laser pump signal and amplifying the input light signal in response to receipt of a laser pump signal at the pre-determined wavelength, there is provided an automatic gain control circuit comprising: a bias voltage device for generating a bias signal for input to the laser pump diode, a center frequency of the laser pump signal being determined by a magnitude of the bias signal; a device for tapping off a portion of the input signal to the optically pumped amplifier; a control circuit for receiving the tapped off input signal portion and generating a control signal for regulating an amount of bias signal applied to the pump laser diode; and, wavelength-locked loop circuit for regulating the amount of optical power delivered to the doped fiber element by enabling real time adjustment of the laser pump signal center wavelength with respect to the optical filter peaked passband center wavelength in accordance with the control signal, wherein automatic gain control is provided according to changing conditions of the input signal.
When implemented to provide automatic gain control or, reduce ASE noise, the wavelength-locked loop servo-control circuit comprises: a mechanism for applying a dither modulation signal at a dither modulation frequency to the laser pump signal, and inputting the dither modulated laser pump signal to the optical filter; a mechanism for converting a portion of the dither modulated laser pump signal to an electric feedback signal; a mechanism for continuously comparing the feedback signal with the dither modulation signal and generating an error signal representing a difference between a frequency characteristic of the feedback signal and a dither modulation frequency; and a mechanism for automatically adjusting the center wavelength of the peaked spectrum function of the laser pump signal according to the error signal, wherein the center wavelength of the laser pump signal and the peaked passband function of the optical filter become aligned when the frequency characteristic of the feedback signal is two times the dither modulation frequency. At this point, maximum amplifier gain level is achieved when providing automatic gain control.
Advantageously, the system and method of the present invention may be employed in optical networks such as WDM and DWDM systems.